This invention relates to a circuit for improving the signal to noise ratio of randomly compressed or uncompressed signals.
Improvement in the signal to noise ratio of recorded signals has always been a consideration in recording-reproducing systems. Processing systems which treat the program material both in recording and playback to improve the signal to noise ratio have been used. Such a processing system which works on playback only is more universal since it can be used on any recorded material. In the past, two approaches have been used to obtain noise reduction in the playback of any recorded material. The first technique is to limit the playback bandwidth to that required by the instantaneous program content. This requires sensing the frequency limits of the program and activating high and low pass filters which allow only the useful program content to go unattenuated. In the absence of signal, the bandwidth is reduced to some minimum value which largely attenuates the noise components of the recording medium. This relatively complex system has not been widely used because the cutting in and out of the filters is often noticeable to the listener.
The second approach is to use an expander to increase the dynamic range of the recorded program, which produces a decrease in background noise by effectively reducing the level of the softer passages relative to the higher level passages and thus reducing the residual noise levels which are most noticeable in the softest parts of the program or in the absence of modulation. An expander circuit generally includes a variable gain amplifier whose gain varies with the amplitude of a direct current (DC) control signal fed thereto from the output of an AC to DC converter circuit fed in parallel with the amplifier by the AC input signal. To be satisfactory, the expansion action must be capable of following the dynamic changes in the program levels accurately both on attack and decay. To accomplish this ideally requires a fast response, low ripple AC to DC converter such as described in my U.S. Pat. No. 3,760,255. Even with such a converter, however, certain types of program material can produce undesirable effects known as "pumping". This occurs when a strong low frequency sound causes the expander to increase its gain. The low frequency sound does not mask high frequency hiss and thus as the amplitude of the low sound changes, the level of the hiss is pumped up and down in conjunction with it. If suitable high frequency sounds are also present they will mask the hiss and the pumping effect will not be noticed. Another instance of pumping occurs when an announcer speaks against a background of applause. His voice causes the expander gain to change which causes noticeable level changes in the applause background. To avoid these effects the frequency spectrum has been divided into low frequency and high frequency channels by means of filters with each channel having separate filters, AC to DC converters and variable gain amplifiers. Each is then expanded separately and then the two parts are recombined. This design is not without drawbacks, since the expansion of a sound and its harmonics into separate channels can distort the relative ratios of the various frequencies, and the complexity of the circuit increases the cost of the equipment involved.